It would make a whole lot of sense for iotools functions to return rainfall in the units that pvlib.soiling expects and snowfall in the units that pvlib.snow expects. However, those modules use different units: rainfall in mm in pvlib.soiling and snowfall in cm in pvlib.snow. Thus we are faced with choosing the lesser of two evils: either we return multiple units from iotools or we burden the user with converting units before running models.

Discussion in #1767 showed tentative support for settling on mm for both types of precipitation and changing the expected units in pvlib.snow. This would be a change that is not easy (or perhaps not possible) to robustly deprecate, so if we are going to make that change, I think it makes sense to squeeze it into 0.10.0. But I'd like to see more support for this change first.

But ask me again tomorrow and I might have a different opinion. (#1767 (review))

@wholmgren now that it's tomorrow, has your opinion changed? ;)